The present invention relates to a sheet feeder in which sheets stacked in a hopper are picked and transported into the sheet feeder, the sheets being fed one by one into the sheet feeder by a separator roller and a brake roller provided in a separating section.
In related sheet feeders used for an image reading apparatuses or the like, a system is widely used in which sheets are separated by making use of a difference between the coefficient of friction between sheets and the coefficient of friction with a separating member (a pad, a belt, or a roller).
In a sheet feeder used in a related image reading apparatus, sheets are taken out one by one by a configuration shown in FIG. 9. In a case where there are a plurality of sheets subject to reading, sheets are piled up, and are set in a hopper so that ends of the sheets are located below a pick roller. As the pick roller is rotated so as to draw the set sheets into a main body of the sheet feeder, the topmost sheet is picked by the frictional force of its portion which comes into contact with the pick roller, and is fed into the sheet feeder. At this time, there are cases where not only the topmost sheet but also a few sheets including the topmost sheet are simultaneously transported. However, an arrangement is provided such that as the thickness of a passable sheet is restricted by a feeding gate, the number of sheets which are fed into the sheet feeder is narrowed down, and only one sheet is fed into the sheet feeder by a separator roller and a brake roller.
Actually, however, since conditions change including such as the use of various types of sheet, there occur problems of faulty feeding in which the sheet cannot be transported into the sheet feeder, and overlap feeding in which a plurality of sheets are fed simultaneously. As a means for realizing a more high-performance sheet feeder, a device setting which is adjusted to the characteristics of the sheets and the setting environment of the device is very effective in terms of the diversity of the sheet characteristics. However, the present situation is such that it is difficult for a user to select a proper device setting, and there are cases where an engineer makes adjustment at the time of installation.
To overcome such problems, the following arts or apparatus are known, among others: an art in which overlap feeding is detected by the sheet thickness immediately after passing a separating section, and if a determination is made that overlap feeding has occurred, the number of reverse rotation of a reverse roller is increased (refer to JP-A-8-188286); an art in which the forwardly or reversely rotating speed of a brake belt is changed in correspondence with a processing speed (refer to U.S. Pat. No. 6,199,854); a sheet transporting device which has a means for adjusting a separating pressure in a case where the moving speed of the sheet is not within a proper range (refer to JP-A-8-188291); and an art in which the load of a braking section is changed in correspondence with the state of movement of a separating section (refer to JP-A-2001-206571).
The reviewing of the mechanism, control, and the separating member of the device itself improves the performance, if viewed from one aspect, but results in the simultaneous occurrence of demerits. For example, if the separating force is enhanced, the overlap feeding can be eliminated, but a jam increases, and the life of expendables is shortened. Furthermore, although in recent years there has been a demand for a feeding device in which different types of sheets can be loaded in a mixed form, if in this case an attempt is made to make adjustment to the characteristics of the sheet, the performance becomes deteriorated in comparison with an identical sheet type since an optimum device setting differs for each sheet.